International peace envoy Kofi Annan has warned that the "specter of all-out war" is increasing in Syria and that President Bashar al-Assad has the first responsibility to end the conflict peacefully.

"The situation is complex and it takes everyone involved in the conflict to act responsibly if the violence is to stop," he said. "But the first responsibility lies in the Syrian government and President Assad."

Annan was speaking in Doha, Qatar, at a ministerial-level meeting of Arab League states on Syria.

The meeting comes in the wake of last week's massacre of more than 100 people in the Syrian city of Houla.

Annan called the massacre in Houla one of many atrocities in Syria whose perpetrators must be brought to account.

"Recently the violence has escalated once again," he said. "The massacres of children, women and men in Al Houla is a terrible crime. Worst of all, it is one of many atrocities to have taken place.

Elusive Cease-Fire

Anna also warned that achieving a true cease-fire remains as elusive as ever even though UN monitors are now on the ground in Syria.

"Today we have 291 UN military observers and over 90 UN civilians in the mission in Syria able to move freely throughout the country and provide objective reports that can inform the Security Council and all of you," he said.

"But we do not have what this was set up to achieve, an end to the appalling violence and abuses and the launch of the political process for a transition that meets the legitimate aspirations of the Syrian people."

Monitors say more than 13,400 people have been killed across Syria since an antiregime uprising erupted in March 2011, including nearly 2,300 since the cease-fire technically went into effect on April 12.

Arab League chief Nabil al-Arabi told the Doha meeting he has sent a letter to the UN Security Council, urging it to take "all necessary measures to protect the Syrian people."

Burhan Ghalioun, head of the Syrian National Council, Syria's largest exile opposition group told the meeting he would welcome Arab military action aimed at ending attacks by government forces against rebels and civilians.

Meanwhile, UN High Commissioner for Human Rights Navi Pillay, the UN's top human rights official, said in Brussels on June 2 that there should be no amnesty for serious crimes committed in Syria.